Mattress



FIG. 1 is a perspective view showing an exemplary embodiment of my new mattress design;

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines are for environmental purposes only and form no part of the claimed design. 

The ornamental design for a mattress, as shown and described. 